Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1998-10-27
2001-07-03
Anderson, Bruce C. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S397000
Reexamination Certificate
active
06255662
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a detector for use with an ion implanter for treating a workpiece such as a silicon wafer.
BACKGROUND ART
One commercially available ion implantation system uses an ion source that includes a source chamber spaced from an implantation chamber where one or more workpieces are treated by ions from the source. An exit opening in the source chamber allows ions to exit the source so they can be shaped, analyzed, and accelerated to form an ion beam. The ion beam is directed along an evacuated beam path to the ion implantation chamber where the ion beam strikes one or more workpieces, typically generally circular wafers. The energy of the ion beam is sufficient to cause ions which strike the wafers to penetrate those wafers in the implantation chamber. In a typical application of such a system the wafers are silicon wafers and the ions are used to “dope” the wafers to create a semiconductor material. Selective implantation with the use of masks and passivation layers allows an integrated circuit to be fabricated.
Rutherford Backscattering Spectroscopy (RBS) is a known technique for analyzing the energy and yield of backscattered ions impinging a target. Known methods of RBS are performed on stationary materials in a laboratory setting for determining the crystalline structure of the material. U.S. Pat. No. 4,967,078 to Purser entitled “Rutherford Backscattering Surface Analyzer with 180-Degree Deflecting and Focusing Permanent Magnet” discloses a RBS analyzer suitable for use in a research laboratory setting. The apparatus of the '078 patent generates a 2.0 MeV ion beam which is directed onto a sample. An RBS detector counts the ions deflected from the sample. Dedicated RBS software then generates a backscattering spectrum from the sample.
Semiconductor materials such as silicon are used in fabricating electronic devices because of their crystalline structure. Materials have a crystalline structure when their atoms are arranged in three dimensions in a regular manner, known as a crystalline lattice. Silicon wafers used in the fabrication of electronic devices are made from large single crystals of silicon specially grown for that purpose. Crystalline structure is advantageous in electronic devices because it facilitates control of the electrical properties of the device and exhibits uniform electrical properties throughout the entire material. Finally, because impurities which degrade device performance tend to collect around irregularities in the atomic structure of a material, regular crystalline structure promotes optimum device performance and yield.
An important parameter of a semiconductor wafer ion implant process is the angle of incidence of the ion beam with respect to the wafer and internal lattice structure of the semiconductor material of the wafer. The angle of incidence is important because of the role it plays in the phenomenon of channeling. Dopant depth profiles vary as a function of position on the wafer surface if the incident angle of the beam varies across the surface. U.S. Pat. No. 5,432,352 to van Bavel entitled “Ion Beam Scan Control” discloses a method for compensating for deviations in the angle of incidence between the ion beam and the semiconductor wafer during ion beam scanning. This patent concerns the use of a variable speed motor which scans the semiconductor wafer through the ion beam. The motor varies the speed of the scan as a function of the predicted angle of incidence between the ion beam and the wafer. Systems such as the one disclosed by van Bavel rely on assumptions about the precision of the wafer cut with respect to the lattice structure of the silicon crystal and assume a perfectly straight ion beam.
DISCLOSURE OF THE INVENTION
The present invention concerns the use of Rutherford Backscattering Spectroscopy to calibrate the orientation of a workpiece support in ion implantation systems. In ion implantation, the relative position of the ion beam to the crystalline structure of the workpiece is important to achieve effective implantation. It is usually not desirable to have an ion beam impact the crystalline lattice structure at a precise 90 degree angle because this causes a phenomenon known as channeling. Channeling degrades the implant profile because it allows implanted ions to penetrate into the workpiece in an unpredictable manner. For the crystalline structure of silicon, ideal implantation occurs when the ion beam impacts the crystalline structure at an angle within a few degrees of 90 degrees, but not exactly 90 degrees. At this orientation, the amount of ions imbedded into the crystalline structure is maximized, while channeling is minimized.
When an ion beam impacts a crystalline structure such as silicon, the number of ions deflected back (“backscattered”) is related to the angle at which the ion beam is impacting the silicon lattice structure. For example, when the beam impacts the lattice structure at exactly 90 degrees, the number of ions deflected back off (“backscattered”) is minimized. In accordance with the present invention, RBS may be used to measure the intensity of backscattered ions to determine the angle of incidence between the ion beam and the silicon wafer. In a production environment, the technique of the present invention may be used to check that the angle of implantation has remained constant and adjustments may be made to a workpiece support orientation if the angle has changed due to crystal cut of the wafers or ion beam path variations.
The use of RBS techniques which measure backscattering of ions from a stationary workpiece are not suitable for calibration for ion implantation processing runs because during a process run, the workpiece support is rotating. This rotation changes the angle of incidence between the beam and the crystalline structure. Without the benefit of the present invention, calibration of the workpiece support for processing runs is made by assuming a given crystalline structure orientation and a perfectly straight ion beam impact. These assumptions introduce error into the calibration procedure. The present invention allows for RBS to be used in calibration of workpiece support position while the support is rotating, thereby directly measuring the angle of incidence between the ion beam and the crystalline structure of the workpiece. The technique greatly improves the accuracy of the positioning of the workpiece relative to the ion beam and enhances the quality of ion implantation.
The present invention includes a process chamber into which one or more workpieces are inserted and mounted on a rotating workpiece support. The orientation of the workpiece support is adjustable around one or two axes. An energy source sets up an ion plasma used to create an ion beam directed at the workpiece. The ion beam impacts the workpiece and ions are either imbedded into the workpiece or are backscattered. An RBS detector measures the intensity of these backscattered ions. During set up or calibration of an ion implantation system, the workpiece is mounted on the support and an ion beam is directed at the workpiece while the workpiece support is rotated as it does during ion implantation. The intensity of the backscattered ions is measured by the RBS detector and the workpiece support orientation is adjusted until a minimum backscattered intensity is found. The minimum backscattered intensity indicates that the workpiece is at an orientation which corresponds to a 90 degree angle of implantation. This orientation is then used as a reference point for subsequent adjustments to the workpiece support to yield a desired angle of implantation. In subsequent set ups or calibration procedures, a new reference point is located using the RBS detector.
An exemplary embodiment of the present invention features a RBS detector which has a rotatable cover with an entry slit which limits the surface of the detector exposed to the backscattered ions. The cover is rotated via a cable to expose new surface areas of the detector as other surfaces are worn out.
Erokhin Yuri
Rubin Leonard Michael
Wilson Shaun Dean
Anderson Bruce C.
Axcelis Technologies Inc.
Watts Hoffman Fisher & Heinke Co., L.P.A.
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